Finder Bug With Work Around

It certainly isn't a universal bug as your post is the first of it's kind. I'm guessing it was a glitch in the MBA. If it happens again with any of your Macs have them do the following;

Boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following:

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

The Finder will remember the previous window state. When you reactivate Finder, it will open a new window using the previous state. If you go to the "View" menu and turn on Sidebar and then do #2 in your list of trouble shooting steps, that should fix it. Also tell your user about this as well. Certain users may be resistant to any kind of training, but such is life.

I can also tell you how this happened. You said, "IT for a Med center". Those kinds of environments have an evil combination of janky, but horribly expensive, apps that could never survive in the real world combined with end users who know 1) enough to be dangerous or 2) nothing. Either some funky IT app or some med student caused Finder to display a window without a sidebar, getting it stuck that way. I think "recents" may actually be then default setting.